1. Field of the Invention
The present invention relates to an immersion type membrane filter apparatus and a method for operating the membrane filter apparatus.
2. Description of the Related Art
Conventionally, solid-liquid separation has been performed in many instances of waste water treatment, collection of valuable substances in water, and like treatments. A solid-liquid separation method employs a membrane filter apparatus which uses a membrane unit composed of a plurality of membrane elements, such as precision filtration membranes or ultrafiltration membranes. In this case, in the membrane element, pressure is applied on the side of water to be treated, or a negative pressure is generated on the side of treated water with respect to each membrane, so that only water permeates through the membranes.
However, in the membrane element, adhesion to the membrane surface of solid matter such as suspended solid causes generation of filtration resistance in addition to resistance intrinsic to the membrane material itself. As adhesion of solid matter proceeds, the associated filtration resistance increases, impairing permeability of the membrane element. In the case of fixed-rate filtration, the pressure difference between untreated water and treated water, i.e., the differential filtration pressure, increases. As a result, energy required for filtration increases. In the case of fixed-pressure filtration, the water permeation rate, i.e., the rate of water permeating through the membrane, decreases. Thus, there is provided a membrane filter apparatus in which the velocity of water flowing along membrane surfaces is increased, or in which the membrane surfaces are cleaned by means of, for example, sponge balls or carriers, thereby minimizing adhesion of solid matter to the membrane surfaces.
In the case of an immersion-type membrane filter apparatus, in which a membrane unit is immersed in water, an aerator is disposed under the membrane element so as to discharge air for aeration of membranes. An air lift action of the discharged air causes a shear force to be applied to membrane surfaces, thereby cleaning the membrane surfaces by means of a mixed flow of air and water (refer to Japanese Patent Publication No. 4-70958).
To uniformly and efficiently supply air discharged from the aerator into gaps between membranes, the membrane unit is disposed within an enclosure, which is open upward and downward, and the aerator is disposed within the enclosure at a lower portion thereof (refer to Japanese Patent Publication No. 7-20592). Alternatively, the membrane unit is disposed within the enclosure, which is open upward and downward, the aerator is disposed within the enclosure at a lower portion thereof, and flow-smoothing means is disposed between the membrane unit and the aerator (refer to Japanese Patent Laid-Open (kokai) No. 8-281080) or a skirt member is disposed at the lower end of the membrane unit (refer to Japanese Patent Laid-Open (kokai) No. 8-281083).
The conventional immersion-type membrane filter apparatus can uniformly supply air discharged from the aerator into each gap between membranes, but involves a problem that, as air rises within each gap between membranes, a flow of bubbles is gradually biased toward a widthwise central portion of the membrane surface.
FIG. 1 shows a schematic view of a conventional immersion-type membrane filter apparatus.
In FIG. 1, numeral 10 denotes a treatment tank for accommodating water to be treated, which is supplied there into through a line L1; numeral 11 denotes a membrane element; numeral 12 denotes a membrane; numeral 13 denotes a frame; numeral 14 denotes a water manifold nozzle attached to the top end of the frame 13; and a line L2 for discharging treated water is connected to the water manifold nozzle 14. A pump P is disposed in the line L2 in order to pump out treated water. A plurality of membrane elements 11 are arrayed adjacent to each other to constitute a membrane unit.
An aerator 15 is disposed under the membrane unit for cleaning the surfaces of the membranes 12, and is connected to an unillustrated air source through a line L3. Air discharged from the aerator 15 is supplied, in the form of bubbles 16, to the membrane element 11 uniformly along an entire bottom end S1 thereof. The bubbles 16, together with water, rise within each gap between the membranes 12. To guide air discharged from the aerator 15 upward, a skirt element 17 is disposed between the membrane unit and the aerator 15.
Since the membrane elements 11 extend along a predetermined length within the treatment tank 10, the opposite side edge portions of the frame 13 and water to be treated present in the vicinity of the edge portions produce resistance to the bubbles 16 which are rising within each gap between the membranes 12. Accordingly, as the bubbles 16 rise within each gap between the membranes 12, the bubbles 16 are gradually biased toward a widthwise central portion of the surface of the membrane 12. As a result, the amount of the bubbles 16 is reduced at the opposite side portions of a top end S2 of each membrane element 11. That is, the bubbles 16 flow at a relatively high rate in a trapezoidal region AR1 and at a relatively low rate in triangular regions AR2 and AR3. Consequently, sludge is removed by action of the bubbles 16 from the surface of the membrane 12 in the region AR1, whereas sludge tends to adhere to the surface of the membrane 12 in the regions AR2 and AR3 due to impairment in the cleaning effect of the bubbles 16, thus failing to clean the entire surface of the membrane 12. Further, the gaps between the membranes 12 are clogged with sludge, SS, colloid, or a like substance, resulting in a failure to maintain good filtration over a long period of time.